1. Field of the Invention
The present invention relates to a signal distribution network from a common branch point to a plurality of user equipment, comprising a distribution unit and a plurality of optical-fibre cables.
The present invention also relates to an optical-fibre cable and to an optical fibre adapted to be used in a signal distribution network.
2. Description of the Related Art
Currently, in the telecommunication field optical technology is mainly used for long-distance transmission of optical signals using the known properties of wide band provided by optical fibres. On the contrary, the most used technology for distributing signals to a plurality of users (such as for example, television and/or analogue and/or digital telephone signals) and for transmitting digital data between electronic equipment (such as for example, the Personal Computers of a LAN network) makes use of electric cables such as, for example, coaxial cables or those consisting of copper pairs.
Nevertheless, electric cables have a relatively narrow band, and they are becoming a bottleneck with respect to the band of signals to be transmitted. Moreover, they exhibit problems of electromagnetic interferences, of impedance matching, and they are difficult to be introduced into the special raceways of a building since they are stiff. In addition, being bulky, they significantly reduce the number of cables that can be inserted into a raceway.
Moreover, due to electrical safety requirements, they require the arrangement of separate raceways from those used for distributing electric energy.
Thus, the research is turning towards the possibility of using optics not just in the long-distance transmission of signals, but also in the signal distribution networks from a common branch point to a plurality of users. In fact, optical-fibre cables are suitable for being inserted into the special raceways of a building since they are not too bulky, they are flexible, light, and free from electromagnetic interferences. Moreover, they are suitable to be inserted into the same raceways used for distributing electric energy. Furthermore, optical fibres potentially have a very wide band, low attenuation values, and they are transparent to the bit rate, to the format and to the transmission code.
Moreover, among the various types of optical fibres, conventional single-mode optical fibres are more preferable than those multi-mode since they are in se less expensive, with lower absorption losses; they are adapted to be used for a wavelength division multiplexing (WDM) transmission and they have a wider band.
Typically, according to the ITU-T G652 standard, conventional single-mode optical fibres have a cutoff wavelength comprised between 1100 and 1280 nm, and they are used with laser sources and detectors operating at about 1300 and/or 1550 nm for the purpose of allowing a transmission in the second or third optical fibre transmission window and a single-mode propagation (at a greater signal wavelength than the cutoff wavelength).
Nevertheless, due to the relatively high cost of opto-elecronic and optical components (such as for example, optical sources and detectors) operating at about 1300 and/or 1550 nm, distribution networks comprising conventional single-mode optical fibres operating in single-mode propagation condition are not very competitive with respect to conventional networks using electrical cables.
Thus, although conventional single-mode optical fibres exhibit several advantages, their use in signal distribution networks to a plurality of users has been strongly limited so far.
For the purpose of overcoming said disadvantages, it has been proposed to implement signal transmission lines with conventional single-mode optical fibres at 1300 and laser sources and detectors operating at about 800 nm, that is, with optical fibres operating in multi-mode propagation condition [G. A. Bogert (“Signal transmission with optical carriers in multimode range of single-mode fibres”, Electronics Letters, January 1987, Vol. 23, No. 2, pages 71-73); F. J. Gillham et al. (“Single mode fiber optic transceiver using short wavelength active devices in long wavelength fiber” SPIE Fiber Networking and telecommunications, 1989, Vol. 1179, pages 26-33); V. C. Y. So et al. (“Multiple wavelength bidirectional transmission for subscriber loop applications”, Electronics Letters, January 1989, Vol. 25, No. 1, pages 16-19) and Ko-ichi Suto et al. (“0.78-μm digital transmission characteristics using 1.3-μm optimized single-mode fiber for subscriber loop” Electronics and Communications in Japan, Part 1, 1992, Vol. 75, No. 2, pages 38-47)].
In fact, said lines allow exploiting the above advantages of single-mode optical fibres and at the same time, reducing the costs as laser sources and detectors operating at about 800 nm are much less expensive than those operating at about 1300 or 1550 nm.
Nevertheless, when used in multi-mode propagation condition, optical fibres exhibit the known phenomenon of intermodal dispersion according to which two different propagation modes (for example, the fundamental mode LP01 and the first higher-order mode LP11) travel at different group velocities, thus causing a temporal broadening of an optical pulse that propagates in fibre.
In an optical-fibre transmission line operating in multi-mode propagation conditions, thus, the intermodal dispersion limits the maximum data transmission speed (that is, the bit rate) or the maximum length of the line.
Some methods have been proposed for the purpose of reducing the intermodal dispersion phenomenon.
M. Romeiser et al. (“Sources and systems: 800 nm transmission on 1300 nm SM fiber”, FOC/LAN '87 & MFOC-WEST, pagg. 388-3891); M. Stern et al. (“Three-channel, high-speed transmission over 8 Km installed, 1300 nm optimised single-mode fibre using 800 nm CD laser and 1300/1500 nm LED transmitters”, Electronics Letters, February 1988, Vol. 24, No. 3, pages 176-177); J. L. McNaughton et al. (“A compact-disc laser system for video single-mode fiber distribution in the subscriber loop ”, FOC/LAN '88, pages 231-233); M. Stern et al. (“Short-wavelength transmission on 1300 nm optimized single-mode fiber”, Optical Engineering, October 1988, Vol. 27, No. 10, pages 901-908) and H. Jorring (“Design of optical fibre for single-mode transmission at 800 nm”, E-FOC/LAN '91, pages 105-108) disclose a local transmission system comprising a conventional optical fibre single-mode at 1300 nm, a laser source (for example, a laser for compact disc or CD) with emission at 800/850 nm and a modal filter for eliminating higher-order modes.
K. A. H. van Leeuwen et al. (“Measurement of higher-order mode attenuation in single-mode fibers: effective cutoff wavelength”, Optics Letters, June 1984, Vol. 9, No.6, pages 252-254) say that a single-mode optical fibre communication system can operate below the theoretical cutoff wavelength of the LP11 mode if the attenuation of the light transmitted in the LP11 mode is sufficiently high to reduce the effects of modal noise and of intermodal dispersion. For this purpose, the Authors introduce a method for determining an attenuation coefficient depending on the wavelength of LP11 mode in a single-mode optical fibre.
K. Kitayama et al. (“Exerimental verification of modal dispersion free characteristics in a two-mode optical fiber”, IEEE Journal of Quantum Electronics, January 1979, Vol. QE-15, No. 1, pages 6-8) disclose the results of theoretical calculations and experimental measures adapted to determine the group delay of LP01 and LP11 modes in a step-index optical fibre along a wavelength region wherein the optical fibre only guides two modes. The results obtained show that there is a wavelength at which the group delays of the two modes coincide.
U.S. Pat. No. 4,955,014 proposes an optical waveguide communication system in the subscriber area wherein the conventional single-mode optical waveguide, optimised for propagation in the range from 1300 to 1600 nm, is used with optical transmitters and receivers whose operating wavelengths are below the waveguide cutoff wavelength. The waveguide is coupled to the laser in such way as to excite a single propagation mode thus allowing a high bit rate digital signal transmission.
U.S. Pat. No. 4,204,745 discloses a graded-index optical fibre having a distribution of the refractive index n as a function of the radial distance r from the core axis, given byn=no[1−Δ(r/a)α]1/2 0≦r≦an=no[1−Δ]=ne r≧awhere no is the refractive index at the core axis, a is the core radius, α is a power exponent, Δ=(no−ne)/ne and ne is the cladding refractive index. In said fibre the power exponent α and the normalised frequency ν [ν=(2πano/λ)*(2Δ)1/2) are selected so that the group delay of the fundamental mode is equal to that of the first higher-order mode.
The Applicant notes that said patent relates to the radiation transmission in multi-mode, and preferably two-mode, propagation condition, in particular at the wavelength of 1.25 μm, and it does not disclose nor it suggests the use of the fibre in single-mode propagation condition.
U.S. Pat. No. 4,877,304 discloses an optical fibre wherein the refractive index no at the core axis, the refractive index of the cladding n1 the core radius a and the core refractive index profile are selected so that: (a) the difference between the normalised delay time of the j-th mode (with j=1 or 2) and the normalised delay time of the fundamental mode is less than about 5*10−2 over a wide range of values of the normalised frequency V [V=(n12−n02)1/2*(2πa)/λ)] and (b) the normalised waveguide dispersion is less than or equal to, 0.2 at V values near the normalised cutoff frequency of the first higher-order mode. In the patent description it is said that, due to the limited number of variables in the design of a step-index refractive profile, or of the α type, fibres having said refractive index profile are not expected to meet both conditions (a) and (b). Examples of fibres capable of meeting said conditions are, for example, those having a segmented core index profile and of the W type. An optical fibre with the above features (a) and (b) can propagate a signal having two or three modes over the wavelength range between 800 and 900 nm with bandwidths comprised between 2 and 4 GHz*Km, and a low dispersion single-mode signal (total dispersion less than 5 ps/Km*nm) at wavelengths greater than 1250 nm.
In the above patent, it is said that said fibre can be used in a certain number of system applications. For example, at first, when the bandwidth requirements are comprised between 2 and 4 GHz*Km, a system using said optical fibre can be operated at wavelengths comprised between 800 and 900 nm wherein the optical fibre guides few modes, so as to exploit the advantage of using low-priced sources and connectors. On the other hand, when at a later time the bandwidth requirements increase, the system can be-upgraded by using terminal equipment operating at higher bit rates, and sources and detectors operating in the low dispersion single-mode region of the optical fibre.
Nevertheless, the Applicant notes that in the practice, the optical fibre disclosed by U.S. Pat. No. 4,877,304 is very difficult and expensive to make. Thus, it is not adapted to be used in an optical fibre distribution network wherein the cost factor is very important.
Jun-ichi Sakai et al. (“Large-core, broadband optical fiber”, OPTICS LETTERS, Vol. 1, No. 5, 1977, pages 169-171) disclose a bimodal broadband optical fibre with larger core diameter than that of a conventional single-mode optical fiber. They state that, by choosing normalized frequency equal to 4.6 and refractive index profile parameter a equal to 4.5, a core diameter as large as 16.3 μm with relative index difference equal to 0.3% at the 1.25 μm wavelength is attainable.